The present invention relates to automatic counting of passing vehicles through the use of video cameras. Throughout this disclosure the words vehicle and car are used interchangeably and are intended to encompass cars as well as trucks of various classes expected in carpark facilities.
Having a notion of occupancy at different levels and zones in multi-story carparks is becoming increasingly desirable for various reasons in many high traffic carparks such as those in airports, sport stadiums, city centers and major malls. Such information can in turn be used to direct drivers more efficiently to unsaturated levels and zones and, in turn, contribute to the carpark throughput. Many other reasons such as fuel consumption, decreasing pollution, or avoiding traffic jams can additionally be cited as benefits of routing drivers more efficiently to empty parking spaces.
A salient characteristic of many multi-story carparks is low ceiling heights compared to the permissible vehicle height. A 7.5 ft story-height where the maximum permissible vehicle height is close to 7 ft is not unprecedented. This application presents many challenges when it comes to isolating the imaged objects from the background and each other—namely, establishing background, foreground and performing image segmentation as well as coping with moving cast shadow and highlights and illumination changes among others.
Use of vision systems for car counting in general traffic and carparks is not unprecedented. For example, Ming-Yee Chiu et al, describe a system entitled “An Embedded Real-time Vision System for 24-hour Indoor/Outdoor”, in the Proceedings of the 17th International Conference on Pattern Recognition” (ICPR04), pp. 338-341. They deploy two camera-test pattern pairs each for one direction, each dedicated to counting cars and trucks in one direction of the two-way roadway. In such arrangement the camera is placed high up on one side of the road while its pairing test pattern sits low, close to the ground on the opposite side of the road. They use the blocking light-beam principle to trigger car counting in the far lane. The test pattern is an elaborate black-and-white radial LED. The test pattern and the far-side vehicle can be obscured by a nearside vehicle particularly if the ceiling height is insufficient, as is often the case in multi-story carparks. On the contrary, the present invention does not depend on viewing a test pattern nor is it hampered by the presence of obscuring near-side vehicles, to be expected in 2-way traffic, as each camera is concerned with counting the nearside vehicle and further tolerates vehicle transgression fully or partially into the wrong lane.
The system relies only on grayscale imagery and does not exploit color. From an image processing view point, the methodology of the present invention makes the following contributions:                9) It introduces a novel pixel-based adaptive background model that always yields a prevailing background, a necessity in the processing sequence of the present invention. As a result, the background model in essence remains unimodal, while supporting two contending distributions at all time. This is markedly different from a bimodal model in whose case much like any multimodal background model all modes co-exist at all time.        10) The above is accompanied by a novel pixel updating scheme.        11) A pixel classification scheme that rests on an adaptive intensity threshold is introduced for assigning the respective pixel coordinate into background or foreground.        12) It resolves the critical step of image segmentation i.e. isolating the passing vehicles—or in general objects—from the background and each other in the spatiotemporal space only in the directions of concern and only to the extent necessary, as opposed to pursing it in a full-fledged manner in the spatial space or spatiotemporal space, yet without resort to any blocking beam scheme including use of test patterns to mark separation of the passing objects; in effect it circumvents the difficult problem of segmenting of objects that may appear to be touching or overlapping;        13) It views the scene through a slit-like monitor zone—i.e. a fraction of the entire image—and consequently reduces the computational load significantly;        14) It does not impose any additional constrain to ambient lighting of multi-story carparks including coping exposure to sunlight;        15) It introduces two novel cast shadow/highlight detectors;        16) It introduces a multitude of motion detection and speed estimation schemes.        